Legislation on vehicle and heavy goods vehicle emissions stipulates, amongst other things, a reduction in the release of nitrogen oxides NOx into the atmosphere. One known way to achieve this objective is to use the SCR (Selective Catalytic Reduction) process which enables the reduction of nitrogen oxides by injection of a reducing agent, generally ammonia, into the exhaust line. This ammonia may derive from the pyrolytic decomposition of an ammonia precursor solution, whose concentration may be the eutectic concentration. Such an ammonia precursor is generally a urea solution.
With the SCR process, the high levels of NOx produced in the engine during combustion at optimized efficiency are treated in a catalyst on exiting the engine. This treatment requires the use of the reducing agent at a precise concentration and of extreme quality. The solution is thus accurately metered and injected into the exhaust gas stream where it is hydrolysed before converting the nitrogen oxide (NOx) to nitrogen (N2) and water (H2O).
In order to do this, it is necessary to equip the vehicles with a tank containing an additive solution (generally an aqueous urea solution) and also a device for conveying and metering the desired amount of additive and injecting it into the exhaust line. Given that the aqueous urea solution generally used for this purpose (eutectic 32.5 wt % urea solution) freezes at −11° C., it is necessary to provide a heating device to liquefy the solution in order to be able to inject it into the exhaust line in the event of starting in freezing conditions. This device should ideally cover at least one part of the storage tank and also the line going from this tank to the injector, and preferably also the accessories encountered in this line (filter, pump, etc.).
In one preferred variant, the subject of Application FR 0755118 (published as FR 2916188) in the name of the Applicant, these accessories are integrated into a base plate submerged in the tank and which is heated using a flexible heater, that is to say one or more resistive track(s) inserted between two flexible films or affixed to a flexible film. Although very effective and capable of matching all the shapes of base plates and of accessories, this type of device has an additional cost and furthermore it does not make it possible to easily heat the inside of large parts such as the pump for example.
However, one type of pump that is very efficient, especially for urea systems, is composed of a rotary pump driven by a brushless direct current (BLDC) motor. In such a motor, the rotor is rotated by the sequential switching (according to a defined timing) of electromagnetic coils placed in the stator. Although the conductive wires at the base of these coils (windings) generally have a relatively low resistance (in order to prevent losses via the Joule effect and excessive heating of the motor), they emit however a not inconsiderable heat, which is of course only available once the pump is started.